1. Field of the Invention
This invention relates to a method of detecting a signal level which has a filtering function. Also, this invention relates to an apparatus for detecting a signal level which has a filtering function.
2. Description of the Related Art
A conventional control system includes a sensor for detecting a condition of a controlled object and outputting an analog signal representative thereof, and a controller for processing the sensor output signal. It is known to provide a low pass filter between the sensor and the controller. The low pass filter removes high-frequency noise from the sensor output signal, and feeds the resultant noise-free signal to the controller. The low pass filter is of the analog type or the digital type.
The analog-type low pass filter has a cutoff frequency determined by the parameters of a resistor and a capacitor. The parameters of the resistor and the capacitor depend on temperature. Therefore, the cutoff frequency of the analog-type low pass filter depends on temperature.
The digital-type low pass filter uses many transistors, and is large in circuit scale and high in cost. The digital-type low pass filter includes an anti-aliasing pre-filter and an A/D (analog-to-digital) converter located before the digital filtering stage. The pre-filter uses an analog-type low pass filter. As previously mentioned, the cutoff frequency of the analog-type low pass filter depends on temperature.
U.S. Pat. No. 5,396,247 discloses a pulse circulating circuit which includes inverting circuits each for inverting an input signal and outputting an inversion of the input signal. A time of signal inversion by each of the inverting circuits varies in accordance with a power supply voltage applied thereto. One of the inverting circuits constitutes a trigger inverting circuit for starting which is controllable in inversion operation. The pulse circulating circuit circulates a pulse signal therethrough after the trigger inverting circuit starts to operate. An input terminal subjected to an analog voltage signal is connected to power supply lines of the respective inverting circuits for applying the analog voltage signal to the inverting circuits as a power supply voltage fed thereto. A counter serves to count a number of times of complete circulation of the pulse signal through the pulse circulating circuit. A circulation position detecting device serves to detect a circulation position of the pulse signal in the pulse circulating circuit on the basis of output signals of the respective inverting circuits. A control device is operative for activating the trigger inverting circuit and thereby starting pulse circulating operation of the pulse circulating circuit, and for activating the circulation position detecting device at a moment which follows a moment of starting pulse circulating operation by a given time. An output device is operative for outputting digital data as a result of A/D conversion of the analog voltage signal. The A/D conversion result data has lower bits composed of output digital data of the circulation position detecting device, and higher bits composed of output digital data of the counter.
It is a first object of this invention to provide a method of detecting a signal level which has a stable filtering function for removing high-frequency noise.
It is a second object of this invention to provide an apparatus for detecting a signal level which has a stable filtering function for removing high-frequency noise.
A first aspect of this invention provides a method of detecting a signal level which has a filtering function. The method comprises the steps of inputting an analog input signal into a pulse delay circuit including a series combination of delay units, the analog input signal controlling signal delay times provided by the respective delay units; inputting a pulse signal into the pulse delay circuit, the pulse signal being transmitted in the pulse delay circuit while being sequentially delayed by the delay units; detecting a number of ones among the delay units through which the pulse signal has passed during a setting time from a moment at which the pulse signal is inputted into the pulse delay circuit; and generating information representative of a level of the analog input signal in response to the detected number.
A second aspect of this invention is based on the first aspect thereof, and provides a method wherein the analog input signal is inputted as a drive voltage for the delay units to control the signal delay times provided by the respective delay units.
A third aspect of this invention is based on the first aspect thereof, and provides a method wherein the signal delay times provided by the delay units depend on drive currents fed thereto respectively, and the analog input signal controls the drive currents fed to the delay units to control the signal delay times provided by the respective delay units.
A fourth aspect of this invention is based on the first aspect thereof, and provides a method further comprising the step of changing the setting time to adjust a low pass filtering characteristic for removing high-frequency noise components.
A fifth aspect of this invention is based on the first aspect thereof, and provides a method wherein the delay units are connected in a ring to form a ring delay line through which the pulse signal circulates, and a position of the pulse signal in the ring delay line is detected and a number of times of complete circulation of the pulse signal through the ring delay line is detected, and the information representative of the level of the analog input signal is generated in response to the detected position and the detected number of times.
A sixth aspect of this invention provides a signal-level detecting apparatus having a filtering function. The apparatus comprises a pulse delay circuit including a series combination of delay units having signal delay times depending on an analog input signal; means for inputting a pulse signal into the pulse delay circuit, the pulse signal being transmitted in the pulse delay circuit while being sequentially delayed by the delay units; means responsive to a clock signal for detecting a position in the pulse delay circuit which the pulse signal reaches at a timing determined by the clock signal; and means for generating information representative of a level of the analog input signal in response to the position detected by the detecting means.
A seventh aspect of this invention is based on the sixth aspect thereof, and provides an apparatus wherein the delay units receive the analog input signal as a drive voltage so that the signal delay times of the delay units depend on the analog input signal.
An eighth aspect of this invention is based on the sixth aspect thereof, and provides an apparatus wherein the pulse delay circuit comprises means for controlling drive currents fed to the delay units in response to the analog input signal to control the signal delay times of the delay units in response to the analog input signal.
A ninth aspect of this invention is based on the sixth aspect thereof, and provides an apparatus wherein the detecting means comprises an encoder for converting output signals of the delay units into a signal representing the position in the pulse delay circuit which the pulse signal reaches.
A tenth aspect of this invention is based on the sixth aspect thereof, and provides an apparatus wherein the delay units are connected in a ring to form a ring delay line through which the pulse signal circulates, and the detecting means comprises means for detecting a number of times of complete circulation of the pulse signal through the ring delay line, and the generating means comprises means for generating the information representative of the level of the analog input signal in response to the detected position and the detected number of times.